1. Field of the Invention
The invention relates to a membrane module for immersed operation with a fibre bundle of a large number of hollow fibre membranes which with an open end are cast into a headpiece and in immersed operation are surrounded by a liquid to be filtered, a permeate collection chamber adjoining the headpiece with at least one permeate outlet for the permeate draining from the interior of the hollow fibre membranes and a gas supply having a pipe guided through the headpiece, wherein the pipe terminates in the interior of the fibre bundle and has a gas outlet for a gaseous medium which, after the transition from the pipe into the liquid to be filtered, largely rises between the hollow fibre membranes of the fibre bundle as bubbles.
2. The Prior Art
Hollow fibre membranes refer to capillary-shaped membranes mostly having a length of more than 1 meter and a diameter in the range between 100 μm and 5 mm. For immersed operation the membrane module is lowered into a basin with a liquid to be filtered wherein the hollow fibre membranes are largely vertically oriented in the liquid. With a membrane module operating in immersed operation the permeate is pumped from the permeate collection chamber. As a result, a pressure is maintained in the permeate collection chamber which is lower than the liquid pressure on the outside of the hollow fibre membranes.
A membrane module with the features described at the outset is known from DE-C 100 45 227. The hollow fibre membranes are cast into the headpiece with one end and individually closed on their other end. The permeate collection chamber with headpiece and gas supply forms a foot part which can be mounted to a frame and for immersed operation is lowered into a basin with the liquid to be filtered. The hollow fibre membranes stand in the liquid to be filtered similar to sea grass and can perform movements relative to one another. Merely excessive movements of the fibre bundle are restricted through lateral fibre retainers. When the membrane module is operated dirt accumulates on the membrane surface of the hollow fibre membranes. Through gassing with air, which is introduced into the interior of the fibre bundle through the gas supply pipe, the liquid to be filtered and the hollow fibre membranes are put in motion and the dirt retained on the hollow fibre membranes can be transported away. However, the known measures are not sufficient to permanently avoid sludge accumulation in the foot area of the fibre bundle. The foot area is a section of the fibre bundle adjoining the headpiece in which the hollow fibre membranes can move only to a limited extent because of their end-sided fixing. The sludge accumulation commencing in the foot area of the membrane module continues upwards and over time results in that a more or less large part of the fibre bundle is completely sludged up and can no longer be rinsed clear even through intensive gassing. The gassing air passes the zones of the fibre bundle with increasing sludge accumulation with the result that the sludge accumulation of the fibre bundle progresses rapidly. Sludge accumulation of the fibre bundle commences the earlier the greater the permeate flow and thereby the local dewatering of the sludge.
A membrane module for immersed operation is known from WO 97/06880 the fibre bundle of hollow fibre membranes of which is fixed between two headpieces. The faces of the two headpieces occupied by hollow fibre membranes are divided into sections while free spaces remain between the sections. The free spaces are used for arranging a gas distributor having channels with gas outlet openings adjoining star-shape to a pipe. The air largely rises in the free spaces of the fibre bundle where it is intended to bring about swirling of the liquid to be filtered. A similar arrangement is described in JP-A 07/185268 for a membrane module, which is not used in immersed operation but in a jacket pipe through which liquid flows. Sludge accumulation of the fibre bundle cannot be permanently avoided even with the arrangements known from WO 97/06880 and JP-A 07/185268.